With a Third Generation Partnership Project (3GPP) Time Division Duplex (TDD) system, time is partitioned into transmission time intervals (TTIs) that are subdivided into frames, which are further subdivided into timeslots. A TTI is defined as one or more radio frames. Specifically, a radio frame is 10 ms; and a TTI may be 10, 20, 40 or 80 ms. The low chip rate TDD divides each frame into two subframes. The subframes are then divided into timeslots. A Coded Composite Transport Channel (CCTrCH) comprises one or more Transport Channels (TrCHs). A CCTrCH is mapped into a collection of one or more sets of timeslots and codes.
When the maximum data size of a CCTrCH is transmitted, all allocated codes and timeslots are used in the TTI. The actual number of codes and timeslots that are transmitted during a TTI are signaled to the receiver via a Transport Format Combination Index (TFCI). Codes and timeslots are allocated according to a set of rules known to both the transmitter and receiver, so once the number of codes and timeslots are known to the receiver by decoding the TFCI, it also knows which codes were transmitted in each timeslot.
A 3GPP TDD system includes support for discontinuous transmission (DTX) of radio frames when the total bit rate of a CCTrCH is less than the total bit rate of the codes and timeslots allocated to the CCTrCH within a TTI. The coding and multiplexing function in a TDD transmitter maps data onto codes and timeslots.
DTX is applied separately to each CCTrCH. When a CCTrCH is in DTX, some or all of the codes and timeslots allocated to the CCTrCH are not transmitted. DTX falls into two categories referred to as partial DTX and full DTX. During partial DTX, a CCTrCH is active but less than the maximum number of codes and timeslots are filled with data, and some codes and timeslots are not transmitted within the TTI. During full DTX, no data is provided to a CCTrCH by upper protocol layers and there is no data at all to transmit within a TTI. A CCTrCH may comprise multiple TrCHs that have different TTIs. In that case, the transmitted codes may change during each interval equal to the shortest TTI among the TTIs for all TrCHs in the CCTrCH. Throughout this document, references to the TTI will mean the shortest TTI among all the TrCHs in the CCTrCH. Since the present invention is directed to full DTX, only full DTX will be described hereinafter.
During full DTX, special bursts (SBs) are transmitted. Each SB is identified by a 0-valued TFCI in the first code of the first timeslot allocated to the CCTrCH. The first SB indicates the start of full DTX. Subsequent SBs are transmitted periodically every Special Burst Scheduling Parameter (SBSP) frame. The subsequent SBs provide a mechanism for the receiver to determine that the CCTrCH is still active, and prevent the receiver from declaring out-of-sync. Full DTX ends when upper protocol layers provide data.
In the 3GPP standard, the MAC entity provides data to the physical layer for transmission. The physical layer generates the SBs, indicating full DTX, whenever the MAC fails to provide any data for transmission. The physical layer ends full DTX reinitiates transmission as soon as the MAC provides data.
The SBSP is known to the transmitter, but not the UE. Hence during full DTX, the UE must process many frames on the possibility that an SB was transmitted, even though SBs are only transmitted once every SBSP frames. Further, the transmitter reinitiates data transmission as soon as data is available from higher layers, and does not synchronize the start of the data transmission with the beginning or end of a sequence of SBSP frames that started with an SB transmission. Hence, the UE must process many frames on the possibility that data transmission has started, even though a CCTrCH may still be in full DTX. Each time the UE turns on to process a frame and look for data or SBs, it uses power. Hence significant power savings for a mobile can be achieved by avoiding the need to turn on during frames when neither an SB nor data is transmitted.